Revert "rpmsg: glink: spi: unregister rpdev during ept destroy"

This reverts commit 3f896899.
There is an incorrect kref_put loop in this change. It results in a
use after free during ssr.

Change-Id: I6620248002db1f2bb71b7325576c95e19d31b6ea
Signed-off-by: Chris Lew <clew@codeaurora.org>